Thyroid autoantibodies in Thai type 1 diabetic patients: clinical significance and their relationship with glutamic acid decarboxylase antibodies.

Objective: To study the clinical significance of thyroid autoantibodies in Thai patients with type 1 diabetes and their relationship with glutamic acid decarboxylase antibodies (GAD(65)Ab).

Methods: Thyroglobulin antibodies (TG-Ab) and thyroid peroxidase antibodies (TPO-Ab) were measured in 50 Thai type 1 diabetic patients. Forty-four patients also had GAD(65)Ab measured. Serum thyrotropin (TSH) was measured in all patients who had no history of thyroid disease regardless of thyroid antibody status. Clinical data including sex, age at onset and duration of diabetes, family history of diabetes, fasting c-peptide levels as well as frequencies of GAD(65)Ab were compared between patients with and without thyroid antibodies. GAD(65)Ab was also measured in 29 non-diabetic patients with hyperthyroid Graves' disease or Hashimoto thyroiditis as a control group.

Results: TG-Ab and TPO-Ab were positive in nine (18%) and 15 (30%) patients, respectively. Eight patients (16%) were positive for both antibodies. Two of 16 patients who were positive for TG-Ab or TPO-Ab had a previous history of hyperthyroidism prior to diabetes onset. Of the remainder, two were newly diagnosed with hyperthyroidism and one was found to have clinical hypothyroidism at the time of the study. None of 34 patients without thyroid antibodies had thyroid dysfunction. Eight patients with positive thyroid antibodies but without clinical thyroid dysfunction and 21 patients without thyroid antibodies were followed for up to 3 years, two patients of the first group developed hypothyroidism, whereas none of the latter developed thyroid dysfunction. The frequency of thyroid dysfunction at the time of initial study was significantly higher in patients with positive thyroid antibodies (3/14 vs. 0/34; P=0.021) and these patients who were initially euthyroid tended to have a higher risk of developing thyroid dysfunction (2/8 vs. 0/21; P=0.069). The frequency of thyroid antibodies was significantly increased in females and in those who had positive GAD(65)Ab. GAD(65)Ab was negative in all of the non-diabetic patients with autoimmune thyroid disease.

Conclusions: About one-fourth of Thai patients with type 1 diabetes without thyroid disease had thyroid antibodies. The frequency of thyroid antibodies was increased in female and in GAD(65)Ab positive patients. The presence of thyroid antibodies is associated with a higher frequency of and may predict a higher risk for thyroid dysfunction in Thai type 1 diabetic patients.